This invention relates to a current sensitive circuit in a field instrument, the circuit for integrating the current entering the circuit, and more particularly, to such circuits which are used in high resolution, continuous measurement applications in the process control industry.
Field instruments include two, three and four wire transmitters which sense a process variable in a process control application and transmit a signal representative of the sensed process variable to a controller. Field instruments also include valve controllers which receive a 4-20 mA current representative of a desired valve position, and provide a pneumatic output as a function of a sensed valve position and the desired valve position. Both transmitters and valve controller operate exclusively on the 4-20 mA current they receive, and can therefore consume only limited amounts of power. Both types of field instruments have analog-to-digital Converters which receive a current from a sensor sensing the process variable and integrate the sensed current in high resolution analog-to-digital converters. Unfortunately, the limited amount of available power and the high degree of resolution required in the instrument mandates that the converters integrate the sensed current over a long integration period, making the measurement suspectable to small leakage currents.
Various types of circuits which cancel, or compensate, for an input offset voltage on an operational amplifier are known. Direct mode cancellation of input offset voltages on an operational amplifier (op-amp) is a commonly used technique in switched capacitor circuits. One type of direct mode cancellation is correlated double sampling, where switches disconnect the voltage input to an amplifier while storing a sample of the offset voltage on a capacitor. After the switches re-connect the voltage input to the amplifier, the stored offset voltage is differenced from the voltage input so that the effective offset is zero. Auto-zeroing techniques, such as the ones described above, are ill-suited for a current measuring application, which only needs to compensate for excessive input leakage current. Additionally, since the voltage input is disconnected from the amplifier in order to provide compensation, this technique is unsuitable for continuous input current sensing circuits.
Another direct mode cancellation technique is chopper stabilization, where the input offset voltage is modulated to a higher frequency and filtered with a lowpass filter. However, the modulation scheme necessitates that the amplifier inputs be disconnected momentarily, rendering this technique also unsuitable for continuous input current sensing.
Two indirect mode techniques for input offset voltage cancellation are also known. In a first technique a current representative of the input offset voltage is created inside the amplifier and a floating gate MOS trim transistor is programmed with a sufficiently large voltage so as to provide enough current to cancel the offset voltage. This technique has inaccuracies because the compensation current created internally does not take into account the temperature and manufacturing process dependencies of the input offset voltage. The other indirect technique uses auxiliary voltage inputs with reduced sensitivity to compensate for amplifier input offset voltage. While this technique provides a continuous time virtual ground and is dynamic, in that the input offset voltage is sensed continuously, this scheme shares the same output as the normal amplifier. A cancellation scheme with shared output such as this overcompensates for the leakage current entering the output, since leakage currents which occur at the input affect the integrator transfer function over a different range of frequencies than do leakage currents occurring at the output. The industry usage of integrators is primarily in the frequencies affected the input leakage current.
There is thus a need for a current cancellation circuit couplable to a transconductance amplifier circuit which substantially cancels the input leakage current but does not compensate for the leakage current at the amplifier's output, where the cancellation circuit operates continuously, is selectively programmable and provides an unaltered operational transconductance amplifier output current.